A dental composite restorative is finding a widespread use as a material for restoring the treated teeth since it is capable of imparting a color tone close to that of natural teeth and is easy to handle. In recent years, the dental composite restorative has been used not only for chiefly treating the anterior teeth but also for restoring the posterior teeth which must bear high occlusal pressures.
Generally, the dental composite restorative is chiefly constituted by a polymerizable monomer, a filler and a polymerization catalyst. The handling property of the paste thereof before being cured, and aesthetic appearance and mechanical properties of the cured product thereof, however, are greatly dependent upon the kind, shape, particle diameter and amount of filling of the filler that is used.
For example, there has heretofore been known a dental composite restorative blended with an inorganic filler having particle diameters of as relatively as several microns or more. This dental composite restorative has a feature in that a cured product thereof exhibits a large mechanical strength accompanied, however, by such problems that the polishing property and wear resistance are poor and that it is not capable of clinically offering a finished surface maintaining luster comparable to that of natural teeth.
In order to solve these problems, there has been proposed the use of inorganic particles having an average particle diameter of not larger than 1 μm and, particularly, the used of inorganic particles having a round shape and/or an inorganic filler of aggregates thereof, contributing to greatly improving the surface luster. With the dental composite restorative using such a fine filler, however, the fine filler has such a very large specific surface area that the paste thereof before being cured exhibits a large viscosity. In order to adjust the viscosity of the paste to such a level that a dentist can use it in the oral cavity, the polymerizable monomer must be blended in inevitably an increased amount causing a decrease in the handling property, an increase in the amount of contraction upon polymerization and a decrease in the mechanical strength.
In order to solve these problems, there has been proposed a method of using an organic/inorganic composite filler as disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 107187/1979. The organic/inorganic composite filler is obtained by mixing a fine inorganic powder and a polymerizable monomer together, polymerizing and curing them, followed by pulverization into particles of diameters of from about several tens to about several hundreds of microns. Use of this organic/inorganic composite filler makes it possible to solve the above-mentioned problem of handling property and Contraction due to the polymerization to some extent while realizing excellent surface smoothness and wear resistance which are the features obtained from the use of the fine filler.
With the dental composite restorative using the above-mentioned organic/inorganic composite filler, however, the color tone after restored is not necessarily in match with the color tone of the natural teeth and, hence, the aesthetic appearance is not satisfactory.
That is, even if there was selected a conventional material that met the color tone or transparency of a patient's teeth, it was not possible to correctly express the color tone or the quality appeal of the teeth, or the boundary between the filled product and the natural tooth became so distinct that a strong unnatural feeling could not be wiped off.
It has generally been said that light diffusion is one of the factors seriously affecting the aesthetic appearance among the optical properties possessed by the dental composite restorative. The light diffusion is a property in that light falling on a semitransparent material like the dental composite restorative, is refracted and reflected by the material filled in the restorative and is diffused into various directions. The reflected and diffused light that is observed acquires a color tone of the semi-transparent material and of a color tone reflecting the background color. It is therefore considered that the higher the light diffusion is, the more the contour is blurred between the natural tooth and the restored material or the background color of the restored material, enabling the color tone to become in match with the natural teeth.
As one of the indexes of light diffusion, there has been proposed a diffusion degree (D) that will be described later. Though it can be said that the light diffusion increases with an increase in the diffusion degree, Japanese Unexamined Patent Publication (Kokai) No. 255516/1997 proposes an art in which the diffusion degree of a dental composite material containing an inorganic filler is adjusted to lie within a predetermined range to adjust the light transmission of the material, thereby to improve aesthetic appearance. However, the above publication teaches that the above object is not accomplished unless there are used two kinds of inorganic fillers, i.e., an inorganic filler having an average particle diameter of not smaller than 1 μm and having a refractive index which is larger, by more than 0.06, than the refractive index of the polymerizable polymer that is cured, and an inorganic filler having an average particle diameter of not larger than 1 μm and having a refractive index which is larger, by not more than 0.06, than the refractive index of the polymerizable polymer that is cured. The above publication, however, does not even describe or suggest the improvement in the aesthetic appearance by adjusting the diffusion degree in a system using the organic/inorganic composite filler. The dental composite restorative containing the organic/inorganic composite filler exhibits optical properties which are different from that of a system that uses the inorganic filler. Therefore, the description of the above publication is not at all helpful in trying to bring the color tone into match with the natural tooth by using the system which contains the organic/inorganic composite filler.
Besides, the dental composite restorative using the above organic/inorganic composite filler is not still satisfactory concerning the handling property and mechanical strength, and further improvement is desired.